New Super Mario said:
Yeah like really, what would they do?
"What would they/he/her do" or any other variation of "How would they fight" is among the worst arguments for a Smash fighter when you already have a race car driver who didn't show anything about a Falcon Punch before the first game (Captain Falcon), a monochrome character from games that predate Mario (Mr. Game and Watch), and a peripheral for the NES (ROB). Sakurai and/or Nintendo will find a way.
In any case, I believe most of the arguments people bring up against the Mii are (Besides just outright hating them). (And I'd like you to know that before you read these. I'm completely neutral on the Mii, I don't care if they're in or not. I'll just be surprised more if they aren't then if they are at this point. There's too many signs pointing toward it already.)
- Miis aren't exactly a historic All-Star like pretty much most of the cast is.
- They might invade the online servers in terms of "With Anyone" mode. As some may only play as Miis either as kids who want to play as themselves, or trolls who might use them to play as Hitler or any other inappropriate Miis
- They feel the Villager already represents the "user avatar" of Smash Bros.
There might be more reasons besides these three that aren't completely biased. But the thing is every single argument against the Mii pretty much have a good counter-argument
Not an all-star? You have to be in crazy denial to not know that Miis were a huge part of how successful the Wii was. And each Smash Bros. celebrates much of what made the previous console generation a success. Maybe not in overall Nintendo history yet, but to the history of the Nintendo Wii. Yes, the Miis are an all-star. Also Wii Fit Trainer probably was never considered an All-Star. And it's quite arguable that Miis are a bigger all-star then the trainer is since they were actually playable in all those Wii games. While in Wii Fit you only saw the Trainer if you did any Yoga or Strength Training stuff.
Invading online? Just find friends that won't play as the Mii to play online against and you have no problems. And about the trolls using Miis for controversial reasons. Well that unfortunately is just an inevitably of bringing Customizable avatars to online games. It's happened with Mario Kart Wii, and I assume there's some with the Wii Sports Club that brings the sports from Wii Sports into online multiplayer capable games. SSB4 would not be the first time this has happened.
Villager already being the user avatar? We don't know yet if it's even possible to customize the Villager yet. They could do something cool like if you connect New Leaf to Smash Bros. you can play as your Villager. But until this is confirmed we don't know if this is a solid argument. And even if that's the case. Not everyone will have New Leaf. While everyone who has a 3DS and/or Wii U should have at least one Mii. It'd be rather stupid if the one customizable character required buying a whole nother game